One can never be sure that a gas transport system always meets its demands. There is always a possibility that some components fail. Because of this a criterion for designing a gas network is that some failure is allowed as long that it does not happen to often. More precisely a maximum unavailability is defined as a design criterion. A failure is defined as : a minimum pressure violation or a gas quality which is outside its band. The behaviour of a system is dependent of the behaviour of its components. The components of interest are compressor stations, mixing stations and supplies. One way of designing a reliable network is to have at least at every compressor station one compressor as spare. Moreover a scenario in which some supplies are degraded has to be used for the planning. This way of planning gives a reliable system, but one does not know how reliable. It can be that money is spent with low benefit, while if it had been spent in an other way it had been far more beneficial. We propose a way to calculate the reliability of a system based on the reliability figures of its components. There are a number of advantages of this approach :
there are no combinations of degradation that are overlooked.
there are no combinations of degradation that are overemphasised.
the components which attribute most to the system failure rate can be identified
the method is straight forward At Gasunie this method is adopted and a program is developed that does the job.
About 30 years ago a large gas field has been discovered in the north of the Netherlands. The N.V. Nederlandse Gasunie was founded in order to buy, transport and sell this gas. Before that time the use of natural gas as a source of energy was very limited in our country. One of the main marketing strategies of the company was reliability. The availability of the energy supply by means of natural gas had to be extremely high under all circumstances. This strategy has proved to be successful. Nowadays about 98% of all houses in the Netherlands are connected to our network. For the technical planning of the transmission system this rather vague notion of "reliability had to be translated to a more concrete design criteria. In the early days of Gasunie the transmission system had a very simple structure. There was only one gas field and from there the pipelines were spreading out over the country. Apart from the pipelines only the compressor stations were the limiting factors for the transport capacity. Problems with this kind of network are only to be expected when the demand is high and since the gas was mainly used for heating this means on very cold winter days.